Electronic circuit for driving LED strings so as to reduce the light flicker

ABSTRACT

LED strings cascaded to one another are driven by an electronic circuit that includes regulation modules and a brightness-compensation module. The regulation modules carry out in sequence a current-regulation phase, in which they regulate the current that flows in the corresponding LED strings. The regulation module includes: a compensation regulator coupled to a compensation LED string and to a capacitor and a generator that generates an electrical quantity indicating the luminous flux emitted by the LED strings and by the compensation LED string. The compensation regulator regulates a current that flows in the compensation LED string as a function of the electrical quantity, discharging the capacitor through the compensation LED string.

PRIORITY CLAIM AND CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Italian Patent Application No.102016000044285, filed on Apr. 29, 2016, the disclosure of which ishereby incorporated by reference in its entirety.

This application is a continuation in part of U.S. application forpatent Ser. No. 15/162,289 filed May 23, 2016, which claims priorityfrom Italian Patent Application No. 102015000089452 filed Dec. 31, 2015,the disclosures of which are incorporated by reference.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application forpatent Ser. No. 15/162,289, filed on May 23, 2016, which claims thepriority benefit of Italian Patent Application No. 102015000089452,filed on Dec. 31, 2015, the disclosures of which are hereby incorporatedby reference in their entirety.

TECHNICAL FIELD

The present invention relates to an electronic circuit for driving lightemitting diode (LED) strings so as to reduce the so-called lightflicker.

BACKGROUND

As is known, LED sources are increasingly widespread, since they arecharacterized, among other things, by a high energy efficiency and a lowpower consumption given the same brightness.

LED sources require driving circuits capable of supplying d.c. currentsat a low voltage. For this reason, in the case where a LED source is tobe supplied through the electric power grid, it is necessary to use,within the driving circuit, a switching converter, such as, for example,a converter of a buck, boost, or flyback type.

Use of switching converters is particularly indicated in the case ofprofessional applications, i.e., in the case of applications in whichthe level of power required is relatively high (for example, higher than50 W), and in which the constraints regarding the packaging andinstallation are not stringent. Instead, in the case of applications(for example) in the domestic field, the powers required are low, andintegration in switching-converter driving circuits is problematic,since the constructional constraints for LED sources, for example asregards the corresponding plugs, are stringent.

As an alternative to the use of switching converters, less complexsolutions have been proposed, also known as AC-LEDs. These solutionspresent some aspects in common, such as: the presence of a rectifiercircuit; the presence of a plurality of LED strings, each string beingformed by a corresponding number of LEDs connected in series; and thepresence of one or more modules, which regulate the current that flowsin the strings as a function of the value of the sinusoidal gridvoltage. An example of driving circuit of an AC-LED type is described inEuropean Patent No. 2645816 (incorporated by reference).

In greater detail, typically a driving circuit of an AC-LED type isconfigured in such a way that, as the sinusoidal grid voltage increases,the number of LED strings connected in series increases, andconsequently the number of LEDs that are on. Further, as the number ofon LEDs increases, the driving circuit increases the regulated current.More in particular, the current increments occur according to discretelevels; the current thus remains constant for a certain time interval,before rising to the next level. The number of current levels is equalto the number of LED strings.

This being said, driving circuits of the so-called AC-LED type areeffectively characterized by a high constructional simplicity. However,they provide only discrete levels of performance as regards flicker ofthe visible radiation generated thereby, on account of the stepwise plotof the current. In this connection, traditionally light flicker isexpressed via two quantities: flicker percent and flicker index. Inparticular, given one period of the supply voltage, the flicker percentis equal to 100·(A−B)/(A+B), where A and B are the maximum and minimumvalues of luminous flux, during the period. Instead, the flicker indexis equal to AREA₁/(AREA₁+AREA₂), where, given an area subtended by thecurve that represents the luminous flux during a period of the supplyvoltage, AREA₁ is the portion of area that exceeds the mean value of theluminous flux in said period, whereas AREA₂ is the portion of area thatis lower than said mean value. In the case of AC-LED systems, typicallyit is possible to obtain values of flicker index and flicker percent of,for example, 0.34 and 99%, respectively.

Since light flicker may be harmful for human health and furtherinterferes with filming and photographing, it is desirable for it to beas low as possible. In order to reduce the flicker index, the U.S. Pat.No. 8,742,682 (incorporated by reference) suggests coupling the LEDstrings to capacitors with high values of capacitance (of the order ofhundreds of microfarads). In this way, however, there occurs a reductionof the service life of the lamp, on account of the reduced service lifeof said capacitors, as well as a deterioration of other characteristicparameters of the AC-LED system, such as efficiency. In addition, thecircuit proposed presents problems of integration.

There is a need in the art to provide an electronic driving circuit thatwill solve at least in part the foregoing drawbacks.

SUMMARY

In an embodiment, an electronic circuit is provided for driving aplurality of LED strings cascaded to one another and subjected to arectified grid voltage. Each LED string forms a respective cathodeterminal. The electronic circuit comprises: a plurality of regulationmodules; and a brightness-compensation module. Each regulation module isconfigured to be coupled to the cathode terminal of a corresponding LEDstring, to a resistive element, and to a capacitor coupled to theresistive element and configured to be charged through at least oneportion of a current that flows in the resistive element, saidregulation modules being further configured to carry out in sequence acurrent-regulation phase, as a function of the evolution of therectified grid voltage. Each regulation module is further configured sothat, when said regulation module carries out the current-regulationphase, it regulates the current that flows in the corresponding LEDstring, in the previous LED strings, and in the resistive element. Thebrightness-compensation module comprises: a compensation regulator,which is coupleable to a compensation LED string, additional to saidplurality of LED strings, and to the capacitor; and a first generatorcoupled to said plurality of regulation modules and to said compensationregulator and configured to generate a first electrical quantityindicating the luminous flux emitted by said plurality of LED stringsand by the compensation LED string. The compensation regulator isconfigured to regulate a current that flows in the compensation LEDstring as a function of said first electrical quantity, discharging thecapacitor through the compensation LED string.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments thereof are now described, purely by way of non-limitingexample with reference to the attached drawings, wherein:

FIG. 1 shows a principle circuit diagram of an optoelectronic circuitincluding the present driving circuit;

FIGS. 2 and 6 show respective circuit diagrams of a first part andsecond part of an embodiment of the present driving circuit;

FIG. 3 shows a circuit diagram of a circuit designed to generate areference voltage;

FIG. 4 shows examples of time plots of signals generated in the circuitshown in FIG. 3;

FIG. 5 shows a circuit diagram of a differential amplifier;

FIG. 7 shows time plots of voltages and currents generated in anembodiment of the present electronic driving circuit;

FIG. 8 shows examples of time plots of currents generated in oneembodiment (not shown), of the present driving circuit, as well as timeplots of a grid voltage, of a total current that flows in saidembodiment, and of an emitted luminous flux;

FIGS. 9 and 10 are respective circuit diagrams of a first part and asecond part of a further embodiment of the present driving circuit; and

FIG. 11 shows examples of time plots of currents generated in anembodiment of the type shown in FIGS. 9 and 10, as well as time plots ofa grid voltage, of a total current that flows in said embodiment, and ofa luminous flux emitted.

DETAILED DESCRIPTION

FIG. 1 shows an optoelectronic circuit 1, which can be electricallycoupled to the power grid 2 through a rectifier 4 formed, for example,by a rectifier of the diode-bridge type, also known as Graetz-bridgerectifier. In this connection, the rectifier 4 comprises a first inputterminal I₁ and a second input terminal I₂, which are each connected tothe power grid 2, and a first output terminal O₁ and a second outputterminal O₂.

The optoelectronic circuit 1 further comprises a plurality of LEDstrings. Purely by way of example, the embodiment shown in FIG. 1comprises a first LED string, a second LED string, a third LED string,and a fourth LED string, designated, respectively, by D1, D2, D3, andD4.

Each LED string is of a per se known type; consequently, albeit notshown in detail, each of the first, second, third, and fourth LEDstrings D1, D2, D3, and D4 may comprise a respective number of LEDs,connected together in series.

In general, the first, second, third, and fourth LED strings D1, D2, D3,and D4 may be different from one another. Further, each of theaforementioned LED strings forms a respective first terminal and arespective second terminal, which will be referred to hereinafter as theanode terminal and cathode terminal. In fact, each LED string isconfigured in such a way that it is traversed by a forward current fromthe respective anode terminal to the respective cathode terminal, onlyif the voltage present between the anode terminal and the cathodeterminal exceeds a corresponding (positive) threshold voltage. In whatfollows, the threshold voltages of the first, second, third, and fourthLED strings D1, D2, D3, D4 will be referred to, respectively, as thefirst, second, third, and fourth threshold voltages V_(th1), V_(th2),V_(th3), V_(th4).

In greater detail, the anode terminal of the first LED string D1 isconnected to the first output terminal O₁ of the rectifier 4. Further,the first, second, third, and fourth LED strings D1, D2, D3, D4 arecascaded in series to one another. In fact, the cathode terminal of thefirst LED string D1 forms a first node N₁, connected to which is theanode terminal of the second LED string D2. The cathode terminal of thesecond LED string D2 forms a second node N₂, connected to which is theanode terminal of the third LED string D3. The cathode terminal of thethird LED string D3 forms a third node N₃, connected to which is theanode terminal of the fourth LED string D4. The cathode terminal of thefourth LED string D4 forms a fourth node N₄.

The optoelectronic circuit 1 further comprises an electronic drivingcircuit 10 and a resistor 12, which will be referred to hereinafter asthe external resistor 12. The external resistor 12 has a resistanceR_(rext), which is, for example, equal to 30Ω.

In FIG. 1, the electronic driving circuit 10 is shown with acorresponding principle block diagram, instead of with the correspondingcircuit diagram, which is represented in FIG. 2 and to which the readeris referred for the details of implementation. This being said, theelectronic driving circuit 10 comprises a circuit module for generationof a reference electrical quantity, which will be referred to in whatfollows as the reference generator 14. Further, the electronic drivingcircuit 10 comprises a control module 16 and a first regulator REG1, asecond regulator REG2, a third regulator REG3, and a fourth regulatorREG4, which are driven by the control module 16 and supply to the latterquantities indicating currents regulated thereby. Further, each of thefirst, second, third, and fourth regulators REG1, REG2, REG3, REG4 has arespective first terminal and a respective second terminal. The firstterminals of the first, second, third, and fourth regulators REG1, REG2,REG3, REG4 are connected, respectively, to the first, second, third, andfourth nodes N₁, N₂, N₃, N₄, whereas the respective second terminals areconnected to a first terminal T1 of the external resistor 12, the secondterminal T2 of which is connected to ground. Further, as shownqualitatively in FIG. 1, the first terminal T1 of the external resistor12 is connected to the control module 16 for enabling a feedbackcontrol, as described in detail hereinafter.

FIG. 1 shows further, once again qualitatively, that the driving circuit10 further comprises a compensation module 17, a fifth regulator REG5, acomparator 18, and a switch 19. Further, the optoelectronic circuit 1comprises a fifth LED string D5, as well as a capacitor 21 and a diode23, which will be referred to, respectively, as the compensationcapacitor 21 and the compensation diode 23.

The compensation capacitor 21 has a value of capacitance of, forexample, 15 μF and is connected between the second terminal T2 of theexternal resistor 12 (i.e., ground) and the cathode of the compensationdiode 23, the anode of which is connected to the second output terminalO₂ of the rectifier 4.

The compensation module 17 is electrically connected to the controlmodule 16, by which it is controlled, as well as to the second terminalT2 of the external resistor 12. Further, the compensation module 17 iselectrically connected to the fifth regulator REG5 for controlling thelatter. In turn, the fifth regulator REG5 has a first terminal and asecond terminal, which are respectively connected to the second terminalT2 of the external resistor 12 and to the anode of the fifth LED stringD5, the cathode of which is connected to the anode of the compensationdiode 23; further, the fifth regulator REG5 is designed to regulate thecurrent that flows in the fifth LED string D5 and supplies to thecompensation module 17 a quantity indicating said current.

The switch 19 is connected between the second terminal T2 of theexternal resistor 12 (i.e., ground) and the second output terminal O₂ ofthe rectifier 4 (and the cathode of the compensation diode 23). Further,when the switch 19 is closed, it connects the second output terminal O₂of the rectifier 4 and the cathode of the compensation diode 23 toground.

For reasons that will be clarified hereinafter, the switch 19 is drivenby the comparator 18, the positive input terminal of which is connectedto the first output terminal O₁ of the rectifier 4. The negative inputterminal of the comparator 18 is, instead, set at a voltageV_(C)+V_(th1), where V_(C) is the voltage drop on the compensationcapacitor 21. As described in greater detail hereinafter, present, inuse, on the first output terminal O₁ of the rectifier 4 is a voltageV_(in); further, the comparator 18 is configured in such a way as: i) toclose the switch 19, when the voltage V_(in) is lower than the voltageV_(C)+V_(th1); and ii) to open the switch 19, when the voltage V_(in) ishigher than the voltage V_(C)+V_(th1). Without any loss of generality,the comparator 18 may comprise a reducer stage (not shown) designed todivide the voltages V_(in) and V_(C)+V_(th1) by a same reduction factorin such a way that the comparison can be carried out on the dividedvoltages.

As shown in greater detail in FIG. 3, the reference generator 14comprises a voltage divider 20, a peak detector 22, a divider 24, amultiplier 26, and a normalization circuit 33.

The voltage divider 20 comprises a pair of resistors 30, 32, which willbe referred to hereinafter as the first and second dividing resistors30, 32. The first terminal of the first dividing resistor 30 isconnected to the first output terminal O₁ of the rectifier 4, whereasthe second terminal of the first dividing resistor 30 is connected tothe first terminal of the second dividing resistor 32, with which itforms a fifth node N₅. The second terminal of the second dividingresistor 32 is connected to ground.

The peak detector 22 comprises a diode 34, the anode of which isconnected to the fifth node N₅, and the cathode of which forms a sixthnode N₆. The peak detector 22 further comprises a capacitor 36 and aresistor 38, which will be referred to in what follows as the outputresistor 38; the capacitor 36 and the output resistor 38 are connectedin parallel between the sixth node N₆ and ground. In practice, the anodeof the diode 34 and the sixth node N₆ form, respectively, the input andthe output of the peak detector 22.

The divider 24 is formed by an electronic circuit of a per se known type(not described in detail), which is designed to generate, on its ownoutput, a voltage signal equal to 1/x, where x is a voltage signalpresent on its own input, as described in greater detail hereinafter.The input of the divider 24 is connected to the output of the peakdetector 22.

The multiplier 26 is formed by a corresponding electronic circuit of aper se known type (not described in detail), which includes a firstinput and a second input and is designed to generate on its own output avoltage signal equal to the product of the voltage signals present onits own first and second inputs. For example, albeit not shown, themultiplier 26 may be formed by a so-called Gilbert multiplier. In thiscase, the divider 24 and the multiplier 26 may be implemented with asingle circuit diagram. Further, the first and second inputs of themultiplier 26 are connected, respectively, to the fifth node N₅ and tothe output of the divider 24. The output of the multiplier 26 isconnected to the input of the normalization circuit 33, the output ofwhich forms a seventh node N₇. In turn, the seventh node N₇ forms theoutput of the reference generator 14.

As mentioned previously, present on the first output terminal O₁ of therectifier 4, and thus at input to the reference generator 14, is thevoltage V_(in), which is formed by a double-halfwave rectifiedsinusoidal voltage and is in phase with the voltage supplied by thepower grid 2. On the fifth node N₅ a voltage V_(part) is thus present,which is equal to k·V_(in), where k is the division ratio introduced bythe voltage divider 20, which may, for example, be 0.0067. Further, onthe sixth node N₆, and thus at output from the peak detector 22, avoltage V_(peak) is present, which is a rectified voltage and has a plotof the type shown in FIG. 4. For simplicity, for the purposes of thepresent description, it is assumed that the voltage V_(peak) is constantand equal to the peak value of the voltage V_(part).

The divider 24 generates a voltage equal to 1/V_(peak), whereas atoutput from the multiplier 26 a voltage equal to V_(part)/V_(peak) ispresent. Further, the normalization circuit 33 is of a per se known typeand is configured to supply on its own output, i.e., on the seventh nodeN₇, a voltage V_(ref)=V_(part)/V_(peak)·V_(nomin), which will bereferred to in what follows as the reference voltage V_(ref). In greaterdetail, V_(nomin) is, for example, equal to 2.1 V.

In practice, the reference voltage V_(ref), supplied by the referencegenerator 14, has the waveform of a double-halfwave rectified sinusoid,in phase with the voltage V_(in) and with an amplitude normalized withrespect to the peak value assumed by the voltage V_(in) itself, in sucha way that, when the voltage V_(in) has a maximum, the reference voltageV_(ref) is equal to V_(nomin). Consequently, the amplitude of thereference voltage V_(ref) is substantially independent of possiblevariations of amplitude of the voltage V_(in), which are caused, forexample, by fluctuations of the voltage supplied by the power grid 2.Consequently, the amplitude of the reference voltage V_(ref) isindependent of the effective peak value of the voltage V_(in). Forsimplicity, in the sequel there is assumed, except where otherwisespecified, operation in nominal conditions, i.e., in the presence of anideal power grid; in these conditions it may be assumed thatV_(ref)=k·V_(in).

Once again with reference to the electronic driving circuit 10, itcomprises a plurality of regulation modules, as shown in detail in FIG.2. In particular, in the embodiment represented in FIG. 2 a firstregulation module MREG1, a second regulation module MREG2, a thirdregulation module MREG3, and a fourth regulation module MREG4 arepresent, which are electrically connected together in sequence, asdescribed hereinafter.

In detail, the first regulation module MREG1 comprises a firstoperational amplifier and a second operational amplifier, designated,respectively, by K1 and W1, as well as a MOSFET M1 and a resistor S1,which will be referred to in what follows as the sensing resistor S1.For example, the MOSFET M1 is of the N-channel enhancement type.

In greater detail, the drain terminal of the MOSFET M1 is connected tothe first node N₁, whereas the source terminal is connected to the firstterminal of the sensing resistor S1, the second terminal of which isconnected to the first terminal T1 of the external resistor 12, thesecond terminal T2 of which, as has been said previously, is connectedto ground.

The gate terminal of the MOSFET M1 is connected to the output terminalof the first operational amplifier K1; the MOSFET M1 is thus driven bythe first operational amplifier K1. The positive input terminal of thefirst operational amplifier K1 is connected to the seventh node N₇,i.e., to the output of the reference generator 14, to be set, in use, atthe reference voltage V_(ref). The negative input terminal of the firstoperational amplifier K1 is connected to the output terminal of thesecond operational amplifier W1, which in use generates a voltageV_(B1), which will be referred to in what follows as the feedbackvoltage V_(B1).

The first regulation module MREG1 further comprises another fourresistors, which will be referred to in what follows as the first,second, third, and fourth adder resistors R_(A1), R_(B1), R_(C1),R_(D1); further, the first regulation module MREG1 comprises adifferential amplifier Z1.

In greater detail, the differential amplifier Z1 is of a per se knowntype and comprises a respective positive input terminal and a respectivenegative input terminal, which are respectively connected to the firstand second terminals of the sensing resistor S1. In use, thedifferential amplifier Z1 generates on its own output (designated byN_(S1)) a voltage V_(S1), which will be referred to in what follows asthe detected voltage V_(S1). The detected voltage V_(S1) is directlyproportional to the current that flows in the sensing resistor S1, andthus to the current that flows in the MOSFET M1.

Purely by way of example, the differential amplifier Z1 may be formed asshown in FIG. 5. In this case, the differential amplifier Z1 comprises arespective operational amplifier 40 and four corresponding resistors,which will be referred to hereinafter as the first, second, third, andfourth additional resistors 42, 44, 46, 48. The first additionalresistor 42 is connected between the output terminal and the negativeinput terminal of the operational amplifier 40. The second additionalresistor 44 has a first terminal connected to the negative inputterminal of the operational amplifier 40, whereas the respective secondterminal forms the negative input terminal of the differential amplifierZ1; thus, it is connected to the second terminal of the sensing resistorS1. The third additional resistor 46 has a first terminal connected tothe positive input terminal of the operational amplifier 40, whereas therespective second terminal forms the positive input terminal of thedifferential amplifier Z1; thus, it is connected to the first terminalof the sensing resistor S1. The fourth additional resistor 48 isconnected between the positive input terminal of the operationalamplifier 40 and ground. By selecting in a per se known manner thevalues of resistance of the first, second, third, and fourth additionalresistors 42, 44, 46, 48 it is thus possible to set the gain (forexample, a unit gain) between the voltage across the input terminals ofthe differential amplifier Z1 and the detected voltage V_(S1), generatedon the output terminal of the differential amplifier Z1. Ideally, thedifferential amplifier Z1 has a common-mode rejection ratio (CMRR) thatis infinite.

Once again with reference to FIG. 2, the second operational amplifier W1forms an adder circuit, together with the first, second, third, andfourth adder resistors R_(A1), R_(B1), R_(C1), R_(D1). In particular,the third adder resistor R_(C1) is connected between the negative inputterminal and the output terminal of the second operational amplifier W1.The fourth adder resistor R_(D1) is connected between the negative inputterminal of the second operational amplifier W1 and ground. The firstand second terminals of the first adder resistor R_(A1) are connected tothe positive input terminal of the second operational amplifier W1 andto the output terminal of the differential amplifier Z1, respectively.The first terminal of the second adder resistor R_(B1) is connected tothe positive input terminal of the second operational amplifier W1,whereas the second terminal of the second adder resistor R_(B1) isconnected to the second regulation module MREG2, as describedhereinafter.

In practice, the aforementioned adder circuit and the first operationalamplifier K1 form, respectively, a first control circuit and a secondcontrol circuit of a control stage designed to control the MOSFET M1.

From a qualitative standpoint, the MOSFET M1, the sensing resistor S1,and the first operational amplifier K1 perform the function of the firstregulator REG1. Further, once again qualitatively, the differentialamplifier Z1 and the adder circuit formed by the second operationalamplifier W1 perform part of the functions of the control module 16.

The second, third, and fourth regulation modules MREG2, MREG3, MREG4 arethe same as the first regulation module MREG1, but for the differencesdescribed hereinafter. Further, given any one of the second, third, andfourth regulation modules MREG2, MREG3, MREG4, the respective electroniccomponents and the voltages generated are denoted by the same terms usedfor the corresponding electronic components/voltages of the firstregulation module MREG1, as well as by the same references, but for thefact that, given any component or voltage of the n-th regulation module,the corresponding reference ends with the number ‘n’, instead of withthe number ‘1’. For this reason, the MOSFET, the first and secondoperational amplifiers, the differential amplifier, the sensingresistor, the feedback voltage, the detected voltage, and the first,second, third, and fourth adder resistors of the second regulationmodule MREG2 are designated, respectively, by M2, K2, W2, Z2, S2,V_(B2), V_(S2), R_(A2), R_(B2), R_(C2), R_(D2), likewise, thecorresponding components/voltages of the third regulation module MREG3are designated, respectively, by M3, K3, W3, Z3, S3, V_(B3), V_(S3),R_(A3), R_(B3), R_(C3), R_(D3); finally, the correspondingcomponents/voltages of the fourth regulation module MREG4 aredesignated, respectively, by M4, K4, W4, Z4, S4, V_(B4), V_(S4), R_(A4),R_(B4), R_(C4), R_(D4). The detected voltages V_(S2), V_(S3), V_(S4) areavailable, respectively, on the outputs of the differential amplifiersZ2, Z3, Z4, which are designated, respectively, by N_(S2), N_(S3), andN_(S4).

Once again with reference to the first regulation module MREG1, theaforementioned second terminal of the second adder resistor R_(B1) isconnected to the output terminal of the second operational amplifier W2of the second regulation module MREG2 for receiving the feedback voltageV_(B2) generated by the latter. Further, the resistance of the firstadder resistor R_(A1) is greater than the resistance of the second adderresistor R_(B1), in such a way that V_(B1)=g1·V_(S1)+g2·V_(B2), withg2>g1, for reasons that will be clarified hereinafter.

Once again with reference to the first regulation module MREG1, thesensing resistor S1 may have a resistance lower than the resistanceR_(rext) of the external resistor 12; for example, the resistance of thesensing resistor S1 may be equal to one thirtieth of the resistanceR_(rext).

Purely by way of example, the sensing resistor S1 may have a resistanceof, for example, 1Ω. The first and second adder resistors R_(A1), R_(B1)may have resistances of, for example, 10 kΩ and 9.8 kΩ, respectively;further, the third and fourth adder resistors R_(C1), R_(D1) may haveresistances of 10 kΩ. In this case, to a first approximation g1=1 andg2=1.01. More in general, the gains g2 and g1 may be close to 1; forexample, there may apply the relations g1=1 and g2=1+Δ, where Δ iscomprised, for example, between 0.01 and 0.1. In addition, thedifferential amplifier Z1 may be configured to amplify the voltage dropon the sensing resistor S1 with a gain equal to unity. In this case, thefirst, second, third, and fourth additional resistors 42, 44, 46, 48may, for example, be the same as one another and, for example, be equalto 10 kΩ. Once again purely by way of example, the first, second, third,and fourth threshold voltages V_(th1), V_(th2), V_(th3), V_(th4) may beapproximately 110 V, 78 V, 60 V, and 40 V, respectively.

As regards the second regulation module MREG2, the drain terminal of therespective MOSFET M2 is connected to the second node N₂. Further, thesecond terminal of the second adder resistor R_(B2) is connected to theoutput terminal of the second operational amplifier W3 of the thirdregulation module MREG3 for receiving the feedback voltage V_(B3)generated by the latter.

As regards the third regulation module MREG3, the drain terminal of therespective MOSFET M3 is connected to the third node N₃. Further, thesecond terminal of the second adder resistor R_(B3) is connected to theoutput terminal of the second operational amplifier W4 of the fourthregulation module MREG4 for receiving the feedback voltage V_(B4)generated by the latter.

As regards the fourth regulation module MREG4, the drain terminal of therespective MOSFET M4 is connected to the fourth node N₄. Further, thesecond terminal of the second adder resistor R_(B4) is connected to thesecond terminal of its own sensing resistor S4, and thus to the firstterminal of the external resistor 12, on which in use there is a voltagedrop V_(rext).

In practice, the positive input terminals of the first operationalamplifiers K1, K2, K3, K4 of the first, second, third, and fourthregulation modules MREG1, MREG2, MREG3, and MREG4 are connected to theoutput of the reference generator 14 and receive the reference voltageV_(ref). Instead, the negative input terminals of the first operationalamplifiers K1, K2, K3, K4 receive the corresponding feedback voltagesV_(B1), V_(B2), V_(B3), V_(B4), which are a function, among otherthings, of the corresponding detected voltages V_(S1), V_(S2), V_(S3),V_(S4). Further, as regards any one of the first, second, and thirdregulation modules MREG1, MREG2, MREG3, the corresponding feedbackvoltage is further a function of the feedback voltage generated by thesubsequent regulation module. In particular, the feedback voltagesV_(B1), V_(B2), V_(B3) are, respectively, a function of the feedbackvoltages V_(B2), V_(B3), V_(B4). As regards the fourth regulation moduleMREG4, the feedback voltage V_(B4) is a function not only of therespective detected voltage V_(S4), but also of the voltage dropV_(rext) on the external resistor 12. On the other hand, since, asexplained previously, the feedback voltages V_(B1), V_(B2), V_(B3) are,respectively, a function of the feedback voltages V_(B2), V_(B3),V_(B4), also the feedback voltages V_(B1), V_(B2), V_(B3) depend uponthe voltage V_(rext) drop on the external resistor 12.

As shown in FIG. 6, the electronic driving circuit 10 further comprisesa first gain circuit Gx1, a second gain circuit Gx2, a third gaincircuit Gx3, and a fourth gain circuit Gx4, the inputs of which areconnected, respectively, to the outputs Ns1, Ns2, Ns3, Ns4 of thedifferential amplifiers Z1, Z2, Z3, Z4 of the first, second, third, andfourth regulation modules MREG1, MREG2, MREG3, MREG4 for receiving,respectively, the detected voltages V_(S1), V_(S2), V_(S3), V_(S4).

In detail, the first, second, third, and fourth gain circuits Gx1, Gx2,Gx3, Gx4 introduce respectively gains gx1, gx2, gx3, gx4, in such a waythat, on the respective outputs, the amplified voltages V_(A1), V_(A2),V_(A3), V_(A4), respectively, are present; there further apply therelations V_(A1)=gx1·V_(S1), V_(A2)=gx2·V_(S2), V_(A3)=gx3·V_(S3), andV_(A4)=gx4·V_(S4). In addition, in what follows it is assumed, forsimplicity, that each of the first, second, third, and fourth LEDstrings D1, D2, D3, D4 is formed by a number NUM of LEDs, and furtherthat gx1=1, gx2=2, gx3=3, gx4=4, for reasons that will be clarifiedhereinafter.

The electronic driving circuit 10 further comprises: a fifth gaincircuit Gx5, a filter 51 of a low-pass type, a first adder stage 53 anda second adder stage 55, a subtractor stage 57, and a MOSFET M5, as wellas a respective differential amplifier Z5 and a respective operationalamplifier K5, which will be referred to, respectively, as the MOSFET M5of the compensation module 17, the differential amplifier Z5 of thecompensation module 17, and the operational amplifier K5 of thecompensation module 17. Further, the electronic driving circuit 10comprises three resistors S5, R_(p1), R_(p2), which will be referred to,respectively, as the sensing resistor S5 of the compensation module 17and the first and second control resistors R_(p1), R_(p2).

In detail, a first terminal of the first control resistor R_(p1) isconnected to the second terminal T2 of the external resistor 12, i.e.,to ground, whereas the second terminal of the first control resistorR_(p1) is connected to a first terminal of the second control resistorR_(p2), with which it forms an eighth node N₈. The second terminal ofthe second control resistor R_(p1) is connected to the anode of thecompensation diode 23.

The MOSFET M5 of the compensation module 17 is, for example, of theP-channel enhancement type. Further, the first and second terminals ofthe sensing resistor S5 are connected, respectively, to the secondterminal T2 of the external resistor 12 and to the source terminal ofthe MOSFET M5 of the compensation module 17, the drain terminal of whichis connected to the anode of the fifth LED string D5. The cathode of thefifth LED string D5 is connected to the anode of the compensation diode23.

The positive input and the negative input of the differential amplifierZ5 of the compensation module 17 are connected, respectively, to thefirst and second terminals of the sensing resistor S5 of thecompensation module 17, in such a way that, in use, the differentialamplifier Z5 generates on its own output a detected voltage V_(S5),which is directly proportional to the current that flows in the sensingresistor S5 of the compensation module 17, and thus to the current thatflows in the MOSFET M5.

The output of the differential amplifier Z5 of the compensation module17 is connected to the input of the fifth gain circuit Gx5, whichintroduces a gain gx5. In what follows, it is assumed, for reasonsdescribed hereinafter, that the fifth LED string D5 is formed by anumber of LEDs equal to 4·NUM, i.e., equal to the total number of LEDsof the first, second, third, and fourth LED strings D1, D2, D3, D4; allthe LEDs are, for example, the same as one another. It is furtherassumed that gx5=4.

On the output of the fifth gain circuit Gx5 an amplified voltageV_(A5)=gx5·V_(S5) is present. Further, the outputs of the first, second,third, fourth, and fifth gain circuits Gx1, Gx2, Gx3, Gx4, Gx5 areconnected to corresponding inputs of the first adder stage 53, theoutput of which is connected to the input of the filter 51 and to afirst input of the subtractor stage 57, a second input of which isconnected to the output of the filter 51.

The output of the subtractor stage 57 is connected to a first input ofthe second adder stage 55, a second input of which is connected to theeighth node N₈.

The positive input of the operational amplifier K5 of the compensationmodule 17 is connected to the output of the subtractor stage 57, whereasthe negative input is connected to the negative input of thedifferential amplifier Z5, and thus to the second terminal of thesensing resistor S5. The output of the operational amplifier K5 isconnected to the gate terminal of the MOSFET M5.

From a qualitative standpoint, the MOSFET M5, the sensing resistor S5,and the operational amplifier K5 perform the function of the fifthregulator REGS. Further, the first, second, third, fourth, and fifthgain circuits Gx1, Gx2, Gx3, Gx4, Gx5, the filter 51, the first andsecond adder stages 53, 55, the subtractor stage 57, the differentialamplifier Z5, and the first and second control resistors R_(p1), R_(p2)form the compensation module 17.

As shown once again in FIG. 6, the electronic driving circuit 10 furthercomprises an estimator circuit 59, which is designed to supply on itsown output a voltage equal to the voltage V_(C)+V_(th1), divided by theaforementioned reduction factor; the output of the estimator circuit 59is connected to the negative input of the comparator 18. The estimatorcircuit 59 is described in detail hereinafter. In general, however,since the reduction factor is introduced already by the estimatorcircuit 59, the comparator 18 does not apply further the reductionfactor on the voltage present on its own negative input, but rather itapplies it only on the voltage present on its own positive input.

Operation of the electronic driving circuit 10 is now described indetail. In order to facilitate understanding, initially the descriptionis limited to a portion of the electronic driving circuit 10 thatincludes the control module 16 and the first, second, third, and fourthregulators REG1, REG2, REG3, REG4; this portion is described withreference to FIG. 7. From another standpoint, the behavior of theelectronic driving circuit 10 is initially described assuming that theswitch 19 is closed and that the compensation module 17, the fifthregulator REG5, the fifth LED string D5, and the compensation capacitor21 and the compensation diode 23 are absent.

This being said, it is assumed that at a first instant t₁ the referencevoltage V_(ref) is zero and that then there follows an ascending portionof the respective profile of the double-halfwave rectified sinusoid.

At the first instant t₁ there cannot flow current in any of the LEDstrings. The voltage drop V_(rext) on the external resistor 12, thedetected voltages V_(S1), V_(S2), V_(S3), V_(S4), and the feedbackvoltages V_(B1), V_(B2), V_(B3), V_(B4) are thus zero. Consequently,each of the first operational amplifiers K1, K2, K3, K4 of the first,second, third, and fourth regulation modules MREG1, MREG2, MREG3, MREG4are in positive saturation, since the voltage on the respective positiveinput terminal (which is equal to the reference voltage V_(ref)) exceedsthe voltage present on the respective negative input terminal (which iszero).

In other words, if V_(M1), V_(M2), V_(M3), V_(M4) are the voltages(shown in FIG. 7) present on the respective output terminals of thefirst operational amplifiers K1, K2, K3, K4 of the first, second, third,and fourth regulation modules MREG1, MREG2, MREG3, MREG4, at the firstinstant t₁ said voltages are equal to a maximum value (which, in theexample shown in FIG. 7, is approximately 7.5 V). Consequently, theMOSFETs M1, M2, M3, M4 of the first, second, third, and fourthregulation modules MREG1, MREG2, MREG3, MREG4 operate in a saturationregion and may be considered as corresponding short circuits.

Next, the increase in voltage V_(in) leads the latter to approximate thefirst threshold voltage V_(th1) of the first LED string D1.Consequently, at a subsequent second instant t₂, a current starts toflow in the first LED string D1, but not in the other LED strings. Inparticular, at the second instant t₂, the voltage V_(in) exceeds thefirst threshold voltage V_(th1).

In practice, if I_(D1), I_(D2), I_(D3), I_(D4) are the currents (shownin FIG. 7) that flow respectively in the MOSFETs M1, M2, M3, M4 of thefirst, second, third, and fourth regulation modules MREG1, MREG2, MREG3,MREG4, starting from the second instant t₂ there occurs an increase ofjust the current I_(D1), which, as has been said, flows, not only in thesensing resistor S1 of the first regulation module MREG1 and in theexternal resistor 12, but also in just the first LED string D1. Theother currents I_(D2), I_(D3), I_(D4) remain zero.

Once a short transient caused by the presence of the series resistancesof the LEDs of the first LED string D1 has vanished, and more preciselystarting from a third instant t₃, the first regulation module MREG1operates in the regulation phase. The regulation phase entails the factthat the first operational amplifier K1 and the MOSFET M1 of the firstregulation module MREG1 have exited from the respective saturationstates, and that the MOSFET M1 operates in the linear region and causesa current to flow in the first LED string D1 that is proportional to thereference voltage V_(ref).

In greater detail, after the third instant t₃, the first regulationmodule MREG1 operates in such a way that the first operational amplifierK1 maintains the voltage between its own positive input terminal(present on which is the reference voltage V_(ref)) and its own negativeinput terminal (present on which is the feedback voltage V_(B1)) zero.More in particular, the first operational amplifier K1, the differentialamplifier Z1, the sensing resistor S1, the MOSFET M1, and the addercircuit including the second operational amplifier W1 form a closedcontrol loop, in which the first operational amplifier K1 operatesoutside saturation, in such a way as to regulate in a linear way thecurrent I_(D1).

Yet in greater detail, since g1≈1 and g2≈1 and the sensing resistor S1has a resistance that to a first approximation is negligible as comparedto the resistance R_(rext) of the external resistor 12, at the thirdinstant t₃ the current I_(D1) is substantially equal to the ratiobetween the voltage V_(rext). and the resistance R_(rext) of theexternal resistor 12. Further, since at the third instant t₃ the voltageV_(rext) is approximately equal (in the aforementioned nominalconditions) to k·V_(th1), where k is the aforementioned division ratiointroduced by the voltage divider 20, we have that the current I_(D1)assumes a value I_(D1) _(_) _(t3)=k·V_(th1)/R_(rext). For example, withk=0.0067, V_(th1)=110 V, and R_(rext)=30Ω, we have that I_(D1) _(_)_(t3) is approximately 25 mA. In this connection, the curves shown inFIG. 7 are provided purely by way of example and refer to a hypotheticalcase, where I_(D1) _(_) _(t3) is approximately 25 mA.

Once again with reference to the third instant t₃, the currents in thesecond, third, and fourth LED strings D2, D3, D4 are zero because thevoltage V_(in) has not yet exceeded the sum of the first and secondthreshold voltages V_(th1), V_(th2), nor, much less, has it exceeded thesum of the first, second, and third threshold voltages V_(th1), V_(th2),V_(th3) or the sum of the first, second, third, and fourth thresholdvoltages V_(th1), V_(th2), V_(th3), V_(th4).

In greater detail, prior to the third instant t₃, the current I_(D1)exhibits a peak, due to the fact that, as explained previously, theMOSFET M1 is in saturation, before the first regulation module MREG1enters the regulation phase. Further, before the regulation module MREG1closes the aforementioned control loop, a time interval elapses, albeitof very limited duration. In what follows, said peak, as likewise thepeaks that occur prior to entry into the regulation phase of the second,third, and fourth regulation modules MREG2, MREG3, MREG4 are notdescribed further, in so far as they are irrelevant for the purposes ofoperation of the electronic driving circuit 10.

This being said, when the first regulation module MREG1 operates in theregulation phase, the current I_(D1) follows a corresponding sinusoidalprofile, as the voltage V_(in) increases. There is thus a linearregulation of the current I_(D1). In particular, the current I_(D1) issubstantially equal to V_(ref)/R_(rext). Likewise, also the voltageV_(M1) follows a corresponding sinusoidal profile; in particular, at thethird instant t₃, the voltage V_(M1) decreases down to a correspondingvalue V_(M1) _(_) _(t3), which depends upon the electricalcharacteristics of the aforementioned control loop, and then follows arespective sinusoidal profile.

In addition, when the first regulation module MREG1 operates in theregulation phase, the first operational amplifiers of the regulationmodules downstream of the first regulation module MREG1, i.e., the firstoperational amplifiers K2, K3, K4 of the second, third, and fourthregulation modules MREG2, MREG3, MREG4 remain in saturation, as likewisethe corresponding MOSFETs.

Following upon increase of the voltage V_(in), at a subsequent fourthinstant t₄, the second regulation module MREG2 enters the regulationphase.

In greater detail, at an instant t₄−δ (with t₃<t₄−δ<t₄) we find that thevoltage V_(in) exceeds the sum of the first and second thresholdvoltages V_(th1), V_(th2), and consequently the current I_(D2) starts toincrease. Further, following upon the fourth instant t₄, the firstoperational amplifier K2 and the MOSFET M2 of the second regulationmodule MREG2 form a closed control loop that regulates the currentI_(D2). In particular, the MOSFET M2 of the second regulation moduleMREG2 operates in the linear region and causes flow in the second LEDstring D2 of a current proportional to the reference voltage V_(ref);further, the first operational amplifier K2 maintains the voltagebetween its own positive input terminal (present on which is thereference voltage V_(ref)) and its own negative input terminal (presenton which is the feedback voltage V_(B2)) zero.

In greater detail, at the fourth instant t₄ the current I_(D2) assumesto a first approximation (in the aforementioned nominal conditions) avalue I_(D2) _(_) _(t4)=k·(V_(th1)+V_(th2))/R_(rext). For example, withk=0.0067, V_(th1)=110 V, V_(th2)=78 V and R_(rext)=30Ω, I_(D2) _(_)_(t4) is approximately 42 mA.

Entry into regulation phase by the second regulation module MREG2entails switching-off of the first regulation module MREG1; i.e., itentails opening of the control loop formed by the first regulationmodule MREG1. In practice, at the fourth instant t₄, the firstoperational amplifier K1 goes into negative saturation, since thevoltage on the respective positive input terminal (equal to thereference voltage V_(ref)) becomes lower than the voltage present on therespective negative input terminal, for the reasons describedhereinafter. In particular, assuming that the first operationalamplifiers K1, K2, K3, K4 are of a unipolar type, the voltage V_(M1)generated on the output of the first operational amplifier K1 becomeszero. Consequently, the MOSFET M1 of the first regulation module MREG1is inhibited and operates as an open circuit. For this reason, followingupon the fourth instant t₄, the current I_(D2) flows in the first andsecond LED strings D1, D2, as well as in the MOSFET M2 and in thesensing resistor S2 of the second regulation module MREG2, but not inthe MOSFET M1 and in the sensing resistor S1 of the first regulationmodule MREG1.

As regards the aforementioned switching-off of the first regulationmodule MREG1, this occurs given that g2>g1, and thus given that, ingenerating the feedback voltage V_(B1), a weight is assigned to thefeedback voltage V_(B2) (and consequently to the detected voltage V_(S2)of the second regulation module MREG2) that is greater than the weightassigned to the detected voltage V_(S1) of the first regulation moduleMREG1. In other words, a gain is applied to the feedback voltage V_(B2),and thus to the detected voltage V_(S2) of the second regulation moduleMREG2, which causes an unbalancing of the voltages present on the inputterminals of the first operational amplifier K1 of the first regulationmodule MREG1. In particular, on the positive input terminal of the firstoperational amplifier K1 of the first regulation module MREG1 thereference voltage V_(ref) is still present, but the feedback voltageV_(B1), present on the negative input terminal, becomes higher than thereference voltage V_(ref).

In greater detail, as mentioned previously, the current I_(D2), which isinitially zero, starts to increase at the instant t₄−δ. At the sametime, the current I_(D1) starts to drop with respect to thecorresponding sinusoidal profile, down to zero at the fourth instant t₄,since the feedback voltage V_(B1) of the first regulation module MREG1depends also upon the detected voltage V_(S2) of the second regulationmodule MREG2.

In practice, in a time interval that has duration equal to δ and thatterminates with the fourth instant t₄, there is passage of current inboth of the MOSFETs M1, M2 of the first and second regulation modulesMREG1, MREG2, in such a way that passage between the phase in whichregulation is carried out by the first regulation module MREG1 and thephase in which regulation is carried out by the second regulation moduleMREG2 occurs without any sharp discontinuity. In particular, in theaforementioned time interval, the first regulation module MREG1 is notyet off (it is outside the saturation region), but no longer regulatesthe current I_(D1) so that the latter is proportional to the referencevoltage V_(ref). Equivalently, in the aforementioned time intervalregulation of the current that as a whole flows in the cascade of theLED strings is assigned to co-operation between the first and secondregulation modules MREG1, MREG2. More in particular, in theaforementioned time interval it is the sum of the currents I_(D1) andI_(D2) that is proportional to the reference voltage V_(ref).

This being said, when the second regulation module MREG2 operates in theregulation phase, the current I_(D2) and the voltage V_(M2) followcorresponding sinusoidal profiles. In particular, the current I_(D2) issubstantially equal to V_(ref)/R_(rext). Further, at the fourth instantt₄, the voltage V_(M2) decreases from the aforementioned maximum valueto a corresponding value V_(M2) _(_) _(t4). In addition, when the secondregulation module MREG2 operates in the regulation phase, the firstoperational amplifiers K3, K4 of the third and fourth regulation modulesMREG3, MREG4 remain in saturation, as likewise the correspondingMOSFETs.

Following upon the further increase in the voltage V_(in), at a fifthinstant t₅ there occur switching-off of the second regulation moduleMREG2 and entry into the regulation phase by the third regulation moduleMREG3; the first regulation module MREG1 remains off. The fifth instantt₅ is subsequent to overstepping, by the voltage V_(in), of the sum ofthe first, second, and third threshold voltages V_(th1), V_(th2),V_(th3).

Following upon the further increase in the voltage V_(in), at a sixthinstant t₆ there occur switching-off of the third regulation moduleMREG3 and entry into the regulation phase by the fourth regulationmodule MREG4; the first and second regulation modules MREG1, MREG2remain off. The sixth instant t₅ is subsequent to overstepping, by thevoltage V_(in), of the sum of the first, second, third, and fourththreshold voltages V_(th1), V_(th2), V_(th3), V_(th4).

Once again with reference to FIG. 7, this shows, purely by way ofexample, also the evolution of the reference voltage V_(ref) and of thefeedback voltage V_(B3) of the third regulation module MREG3. Inpractice, it may be noted how the feedback voltage V_(B3) is lower thanthe reference voltage V_(ref) until the fifth instant t₅, withconsequent positive saturation of the first operational amplifier K3 ofthe third regulation module MREG3. Between the fifth and the sixthinstants t₅, t₆, the feedback voltage V_(B3) is equal to the referencevoltage V_(ref), since at the fifth instant t₅ the third regulationmodule MREG3 has entered the regulation phase. At the sixth instant t₆,the feedback voltage V_(B3) exceeds the reference voltage V_(ref). Thus,the third regulation module MREG3 turns off.

Following upon the sixth instant t₆, the voltage V_(in) assumes arespective maximum value and then starts to decrease. In particular, ata seventh instant t₇, the voltage V_(in) becomes lower than the sum ofthe first, second, third, and fourth threshold voltages V_(th1),V_(th2), V_(th3), V_(th4). Consequently, the current I_(D4) vanishes.

In detail, the current I_(D4) tends to drop prior to the seventh instantt₇, on account of the presence of the series resistances of the LEDs ofthe LED strings. This means that, at an instant t₇−ε, the fourth controlmodule MREG4 exits from the regulation phase.

In greater detail, at the instant t₇−ε, the reference voltage V_(ref)present on the positive input terminal of the first operationalamplifier K4 becomes greater than the feedback voltage V_(B4) present onthe negative input terminal. Consequently, the first operationalamplifier K4 of the fourth regulation module MREG4 goes into positivesaturation. At the same time, the current I_(D3) starts to increase.Further, since the contribution to the feedback voltage V_(B3) of thethird regulation module MREG3 due to the detected voltage V_(S4) of thefourth regulation module MREG4 has vanished, the feedback voltage V_(B3)equals the reference voltage V_(ref). Consequently, at the seventhinstant t₇, the third regulation module MREG3 returns into theregulation phase.

At a subsequent eighth instant t₈, the voltage V_(in) becomes lower thanthe sum of the first, second, and third threshold voltages V_(th1),V_(th2), V_(th3); consequently, the current I_(D3) vanishes. Before thecurrent I_(D3) vanishes, the first operational amplifier K3 of the thirdregulation module MREG3 goes into positive saturation. Further, at theeighth instant t₈, the second regulation module MREG2 returns into theregulation phase.

Likewise, at a ninth instant t₉ the voltage V_(in) becomes lower thanthe sum of the first and second threshold voltages V_(th1), V_(th2);consequently, the current I_(D2) vanishes. Before the current I_(D2)vanishes, the first operational amplifier K2 of the second regulationmodule MREG2 goes into positive saturation. Further, at the ninthinstant t₉, the first regulation module MREG1 returns into theregulation phase.

Finally, at a tenth instant t₁₀, the voltage V_(in) becomes lower thanthe first threshold voltage V_(th1); consequently, the current I_(D1)vanishes. Before the current I_(D1) vanishes, the first operationalamplifier K1 of the first regulation module MREG1 goes into positivesaturation; thus, the first regulation module MREG1 exits from theregulation phase.

In practice, the present electronic driving circuit 10 comprises aplurality of regulation modules electrically connected in sequence, eachof which electrically couples to the cathode terminal of a correspondingLED string. The regulation modules are configured to carry out in turnone current-regulation phase. Further, the current-regulation phasesoccur in a pre-set sequence, as a function of the plot of the referencevoltage V_(ref). In particular, when the amplitude of the referencevoltage V_(ref) is increasing, the first, second, third, and fourthregulation modules MREG1, MREG2, MREG3, MREG4 carry out the respectiveregulation phases in succession, i.e., in a first order. Instead, whenthe amplitude of the reference voltage V_(ref) is decreasing, the first,second, third, and fourth regulation modules MREG1, MREG2, MREG3, MREG4carry out the respective regulation phases in a second order, oppositeto the first order. In addition, each regulation module is such that,when it operates in the current-regulation phase, it regulates thecurrent that flows in the corresponding LED string and in the previousLED strings in such a way that said current is directly proportional tothe reference voltage V_(ref).

All this being said, there follows a detailed description of operationof the entire electronic driving circuit 10, i.e., where theaforementioned hypothesis as regards closing of the switch 19 andabsence of the compensation module 17, of the fifth regulator REG5, ofthe fifth LED string D5, of the compensation capacitor 21, and of thecompensation diode 23 has been removed.

In detail, since, as mentioned previously, the fifth LED string D5 isformed by a number of LEDs equal to the total number of LEDs of thefirst, second, third, and fourth LED strings D1, D2, D3, D4, it may beshown that the second terminal T2 of the external resistor 12 is set ata voltage V_(MID), which exhibits variations in time, around a valueapproximately equal to V_(inpk)/2, where V_(inpk) is the peak value ofthe voltage V_(in).

In greater detail, assuming that the voltage V_(in) is increasing afterit has exhibited a respective zero, it is found that, as long asV_(in)<V_(C)+V_(th1), the switch 19 is closed, and thus the secondterminal T2 of the external resistor 12 is shorted with the secondoutput terminal O₂ of the rectifier 4. Consequently, if by “ordinaryportion” is meant the aforementioned portion of the electronic drivingcircuit 10 that includes the control module 16 and the first, second,third, and fourth regulators REG1, REG2, REG3, REG4, it may be shownthat the ordinary portion operates according to what is described withreference to FIG. 7. In other words, the first, second, third, andfourth regulation modules MREG1, MREG2, MREG3, MREG4 enter in sequencethe regulation phase; further, if I_(Dx) is the current that at ageneric instant enters the first terminal T1 of the external resistor12, it flows towards the second output terminal O₂ of the rectifier 4,without flowing through the fifth LED string D5, on account of thepresence of the compensation diode 23.

If by “compensation portion” is meant the portion of electronic drivingcircuit 10 including the first, second, third, fourth, and fifth gainstages Gx1, Gx2, Gx3, Gx4, Gx5, the filter 51, the first and secondadder stages 53, 55, the subtractor stage 57, the first and secondcontrol resistors R_(p1), R_(p2), as well as the MOSFET M5, thedifferential amplifier Z5, the operational amplifier K5 and the sensingresistor S5 of the compensation module 17, it is found that saidcompensation portion operates in such a way as to reduce the variationsof the luminous flux of the radiation emitted by the optoelectroniccircuit 1.

In greater detail, as long as V_(in)<V_(C)+V_(th1), the compensationcapacitor 21 discharges through the sensing resistor S5 and the MOSFETM5 of the compensation module 17, and consequently also through thefifth LED string D5. In this connection, in fact, it is assumed that theresistance of the sensing resistor S5 is negligible as compared to theresistances of the first and second control resistors R_(p1), R_(p2).For example, the resistances of the first and second control resistorsR_(p1), R_(p2) may be, respectively, 7.5 kΩ and 1.5 MΩ. As regards,instead, the resistance of the sensing resistor S5, it may be equal, forexample, to the resistance of the external resistor 12, which, as hasbeen said previously, is, for example, 30Ω. In this case, assuming forinstance that the gains of the differential amplifiers of the first,second, third, and fourth regulation modules MREG1, MREG2, MREG3, MREG4are equal to unity and that the corresponding sensing resistances are,as mentioned previously, 1Ω, the differential amplifier Z5 of thecompensation module 17 may introduce a gain equal to 1/30. In fact, inregard to the compensation module 17, the sensing resistor S5 performsthe functions of sensing resistor and external resistor; consequently,in order to estimate correctly the various contributions of luminousflux, it is possible equalize the gain introduced by the pair formed bythe sensing resistor S5 and the differential amplifier Z5 with the gainintroduced by one of the corresponding pairs of regulation modules,which are the same as one another.

Since the detected voltages V_(S1), V_(S2), V_(S3), V_(S4), V_(S5) areproportional to the currents that flow in the MOSFETs M1, M2, M3, M4,M5, respectively, the sum of the amplified voltages V_(A1), V_(A2),V_(A3), V_(A4), V_(A5) is directly proportional to the luminous fluxemitted as a whole by the optoelectronic circuit 1, i.e., by the first,second, third, fourth, and fifth LED strings D1, D2, D3, D4 and D5. Thisresult also derives from the fact that, when the current regulation iscarried out by the n-th regulation module, the current flows both in then-th LED string and in the previous LED strings; for this reason, thegains gx1, gx2, gx3, gx4, gx5, which are applied to the detectedvoltages V_(S1), V_(S2), V_(S3), V_(S4), V_(S5), and thus to thecorresponding currents, satisfy the following criterion: they aredirectly proportional, through a same factor of proportionality (whichmay be any), to the number of LEDs that are traversed by thecorresponding current.

This being said, the first adder stage 53 generates a voltage V_(Itot),which is directly proportional to the sum of the amplified voltagesV_(A1), V_(A2), V_(A3), V_(A4), V_(A5), and thus is directlyproportional to the luminous flux emitted as a whole by theoptoelectronic circuit 1.

The voltage V_(Itot) is filtered by the filter 51, which generates avoltage V_(Imed), directly proportional to the mean value of theluminous flux emitted as a whole by the optoelectronic circuit 1.

The subtractor stage 57 generates a voltage V_(IΔ), equal to thedifference between the voltage V_(Itot) and the voltage V_(Imed). Thevoltage V_(IΔ) is thus directly proportional to the instantaneousdeviation of the luminous flux emitted as a whole by the optoelectroniccircuit 1 with respect to the mean value of said luminous flux.

The second adder stage 55 generates a voltage V_(fb), which is directlyproportional to the sum of the voltage V_(IΔ) and a voltage V_(N8),present on the eighth node N₈, which is directly proportional to thevoltage present on the second terminal T2 of the external resistor 12.

In practice, the current that flows in the fifth LED string D5, and thusalso the voltage present on the second terminal T2 of the externalresistor 12, is regulated by means using a first control loop, whichenvisages generation of the voltage V_(N8) and control of the voltagepresent on the gate terminal of the MOSFET M5 as a function of thevoltage V_(N8), and a second control loop, which envisages generation ofthe voltage V_(IΔ) and control of the voltage present on the gateterminal of the MOSFET M5 as a function of the voltage V_(IΔ).

In particular, the first control loop guarantees that the voltageV_(MID) present on the second terminal T2 of the external resistor 12will vary in time around a value approximately equal to V_(inpk)/2, thuspreventing said voltage from assuming values that are too high or toolow, which would cause, respectively, failure to switch on of one ormore of the LED strings D1, D2, D3, D4 and the impossibility ofswitching on the LED string D5. The second control loop renders,instead, the luminous flux emitted as a whole by the optoelectroniccircuit 1 practically constant, limiting the variations thereof (i.e.,minimizing the variations of the luminous flux, compatibly withoperation of the first control loop), thus considerably reducing thelight flicker. The weights with which the voltages V_(IΔ) and V_(N8) aregenerated, starting respectively from the voltage V_(MID) and from theensemble of the detected voltages V_(S1)-V_(S5), are such that the firstcontrol loop is dominant with respect to the second control loop;consequently, the reduction of light flicker is obtained after theoptoelectronic circuit 1 has reached a respective stable working point.

When V_(in)>V_(C)+V_(th1), the switch 19 is opened. In this connection,it may be noted how the switch 19, as likewise the MOSFET M5 of thecompensation module 17, may be formed, for example, by a P-channelenrichment MOSFET. It may here be pointed out how the structure of theestimator circuit 59 is irrelevant for the purposes of operation of theoptoelectronic circuit 1. For example, albeit not shown, the estimatorcircuit 59 may be electrically coupled to the first output terminal O₁of the rectifier 4, to the output of the differential amplifier Z1 ofthe first regulation module MREG1, and to the second terminal T2 of theexternal resistor 12; further, the estimator circuit 59 may beconfigured to sample the voltage V_(in) when the voltage V_(S2) is otherthan zero (i.e., when the voltage V_(in) is approximately equal to thefirst threshold voltage V_(th1)), to add the value sampled to thevoltage present on the second terminal T2 of the external resistor 12,and then to apply the aforementioned reduction factor.

Once again with reference to the moment when V_(in)>V_(C)+V_(th1), atthis instant the switch 19 opens.

As shown in greater detail in FIG. 2, if V_(po) is the voltage betweenthe first output terminal O₁ of the rectifier 4 and the second terminalT2 of the external resistor 12, immediately prior to opening of theswitch 19 said voltage V_(po) is equal to V_(C)+V_(th1); following uponopening of the switch 19, the voltage V_(po) assumes a value equal toV_(in)−V_(C) (equal to V_(th1)), and thus undergoes a sensiblereduction. Notwithstanding the reduction, the voltage V_(in) has by nowachieved a value such as to enable in any case regulation of the currentby the first regulation module MREG1.

Yet in greater detail, immediately prior to opening of the switch 19,the fourth regulation module MREG4 regulates the current that flows inthe first, second, third, and fourth LED strings D1, D2, D3, D4, whichis directly proportional to the voltage V_(in). Following upon openingof the switch 19, regulation is entrusted to the first regulation moduleMREG1, whereas the current flows only in the first LED string D1 andcontinues to be directly proportional to the voltage V_(in); next, thefurther increase in the voltage V_(in) again leads to entry into theregulation phase, in succession, of the second, third, and fourthregulation modules MREG2, MREG3, MREG4. In other words, for each step inwhich the voltage V_(in) is (for example) increasing, each regulationmodule carries out two regulation phases. In this way, an improvement ofthe power factor (PF) and of the total harmonic distortion (THD) isguaranteed, since the current follows the voltage V_(in) more closely.

During the time interval when V_(in)>V_(C)+V_(th1), the compensationcapacitor 21 can recharge, since the current that flows in the fifth LEDstring D5 is lower than when V_(in)<V_(C)+V_(th1), because the first,second, third, and fourth LED strings D1, D2, D3, D4 are fullyconducting and provide a greater contribution to the overall luminousflux of the optoelectronic circuit 1.

FIG. 8 regards a possible embodiment (not shown), in which the fourthregulation module MREG4 is absent. Further, FIG. 8 shows, purely by wayof example, the time plots of the currents I_(D1)*, I_(D2)*, I_(D3)*that flow respectively in the first, second, and third LED strings D1,D2, D3, as well as the current I_(DCOMP) that flows in the fifth LEDstring D5; in addition, FIG. 8 shows the plots of the grid voltage, ofthe total current that enters the anode of the first LED string D1, andof the luminous flux emitted as a whole by the optoelectronic circuit 1.

In practice, the embodiment described previously envisages control ofthe fifth LED string D5 in a way complementary with respect to thefirst, second, third, and fourth LED strings D1, D2, D3, D4, and as afunction of the luminous flux emitted as a whole by the optoelectroniccircuit 1, which to a first approximation no longer depends upon theevolution of the grid voltage. In other words, the fifth LED string D5is driven in phase opposition with respect to the first, second, third,and fourth LED strings D1, D2, D3, D4 so as to reduce the light flicker.

In yet other words, the compensation capacitor 21 can function asenergy-storage element, when V_(in)>V_(C)+V_(th1); instead, whenV_(in)<V_(C)+V_(th1), the charge stored by the compensation capacitor 21is yielded in order to control the fifth LED string D5 forcounterbalancing variations of the luminous flux emitted by the first,second, third, and fourth LED strings D1, D2, D3, D4.

In addition, the electronic driving circuit 10 operates in such a waythat the charge supplied to the compensation capacitor 21, whenV_(in)>V_(C)+V_(th1), is equal to the charge that flows through thefifth LED string D5, when V₁<V_(C)+V_(th1).

Albeit not shown, further possible are embodiments of the type describedwith reference to FIGS. 2 and 6, but in which the switch 19, thecomparator 18, and the estimator circuit 59 are absent. Operation ofthese embodiments is substantially the same as what has been describedpreviously. However, these solutions have a lower performance as regardspower factor and total harmonic distortion. In fact, conduction of thefirst, second, third, and fourth LED strings D1, D2, D3, D4 is possibleonly when V_(in)>V_(C)+V_(th1); this means that, for each half-cycle ofthe grid voltage, the current that flows in the LED strings will remainzero for a non-negligible time.

FIGS. 9 and 10 show a different embodiment, which is now described withreference to the differences with respect to the embodiment shown inFIGS. 2 and 6. Electronic components and voltages already present in theembodiment shown in FIGS. 2 and 6 are denoted by the same terms and thesame references, except where otherwise specified.

In detail, as shown in FIG. 9 and without any loss of generality, thefourth regulation module MREG4 is absent. Further, with reference to thefirst regulation module, here designated by MREG1′, it once againincludes the first operational amplifier K1 and the MOSFET M1, which isdriven by the first operational amplifier K1; in addition, the firstregulation module MREG1′ comprises a respective switch SW1 and a firstresistor R_(MA1), a second resistor R_(MB1), and a third resistorR_(MC1), which will be referred to in what follows as the first, second,and third regulation resistors R_(MA1), R_(MB1), R_(MC1), respectively.

In greater detail, a first terminal of the first regulation resistorR_(MA1) is set at a reference voltage V_(DD), which is for example 3.3V. The second terminal of the first regulation resistor R_(MA1) isconnected to a first terminal of the second regulation resistor R_(MB1),with which it forms a node N_(RA1), which will be referred to as thefirst regulation node N_(RA1). The second terminal of the secondregulation resistor R_(MB1) is connected to a first terminal of thethird regulation resistor R_(MC1), with which it forms a node N_(RB1),which will be referred to as the second regulation node N_(RB1); thesecond terminal of the third regulation resistor R_(MC1) is connected toground.

The source terminal of the MOSFET M1 is connected to the negative inputof the first operational amplifier K1 and to the first terminal T1 ofthe external resistor 12.

The switch SW1 is pre-arranged for connecting the positive input of thefirst operational amplifier K1 alternatively to the first regulationnode N_(RA1) or to the second regulation node N_(RB1), as a function ofa voltage V_(GSW), which is present on the output of the comparator 18.More in particular, the switch SW1 connects the positive input of thefirst operational amplifier K1 respectively to the second regulationnode N_(RB1), when V_(in)<V_(C)+V_(th1), and to the first regulationnode N_(RA1), when V_(in)>V_(C)+V_(th1).

Once again with reference to FIG. 9, the second and third regulationmodules (here designated by MREG2′ and MREG3′) are the same as the firstregulation module MREG1′, but for the differences described hereinafter;further, given any one of the second and third regulation modulesMREG2′, MREG3′, the respective electronic components and the voltagesgenerated are denoted by the same terms used for the correspondingelectronic components/voltages of the first regulation module MREG1′, aswell as by the same references, but for the fact that, given anycomponent or voltage of the n-th regulation module, the correspondingreference ends with the number ‘n’, instead of with the number ‘1’.

In detail, the switch SW2 of the second regulation module MREG2′ iscontrolled by the voltage V_(GSW) and operates in such a way as toconnect the positive input of the first operational amplifier K2 of thesecond regulation module MREG2′ to the second regulation node N_(RB2) ofthe second regulation module MREG2′, when V_(in)<V_(C)+V_(th1), and tothe first regulation node N_(RA2) of the second regulation moduleMREG2′, when V_(in)>V_(C)+V_(th1).

The switch SW3 of the third regulation module MREG3′ is controlled viathe voltage V_(GSW) and operates in such a way as to connect thepositive input of the first operational amplifier K3 of the thirdregulation module MREG3′ to the second regulation node N_(RB3) of thethird regulation module MREG3′, when V_(in)<V_(C)+V_(th1), and to thefirst regulation node N_(RA3) of the third regulation module MREG3′,when V_(in)>V_(C)+V_(th1).

The first, second, and third regulation resistors R_(MA1), R_(MB1),R_(MC1) of the first regulation module MREG1′ have values of resistanceof, for example, 8800Ω, 1200Ω, and 6923Ω, respectively.

The first, second, and third regulation resistors R_(MA2), R_(MB2),R_(MC2) of the second regulation module MREG2′ have values of resistancesuch that the first regulation node N_(RA2) of the second regulationmodule MREG2′ is at a voltage higher than the voltage of the secondregulation node N_(RB1) of the first regulation module MREG1′. Forexample, the first, second, and third regulation resistors R_(MA2),R_(MB2), R_(MC2) of the second regulation module MREG2′ have values ofresistance of, for example, 7700Ω, 2300Ω, and 8333Ω, respectively.

The first, second, and third regulation resistors R_(MA3), R_(MB3),R_(MC3) of the third regulation module MREG3′ have values of resistancesuch that the first regulation node N_(RA3) of the third regulationmodule MREG3′ is at a voltage higher than the voltage of the secondregulation node N_(RB2) of the second regulation module MREG2′. Forexample, the first, second, and third regulation resistors R_(MA3),R_(MB3), R_(MC3) of the third regulation module MREG3′ have values ofresistance of, for example, 6500Ω, 3500Ω and 9500Ω, respectively.

In practice, the first, second, and third regulation modules MREG1′,MREG2′ and MREG3′ operate in sequence, regulating the current in adiscrete way, instead of a continuous way, as described in greaterdetail hereinafter.

As regards the compensation portion of the electronic driving circuit10, it is shown in FIG. 10 and is now described with reference only tothe differences with respect to what is shown in FIG. 6.

In detail, the fourth gain circuit Gx4 is absent; further, the inputs ofthe first, second, and third gain circuits Gx1, Gx2, Gx3 are connectedto the first terminal T1 of the external resistor 12. By way of example,the gains of the first, second, and third gain circuits Gx1, Gx2, Gx3are gx1=1, gx2=2, gx3=3, respectively. Further, it is assumed that thefifth LED string D5 is formed by a number of LEDs equal to 3·NUM, i.e.,is equal to the total number of LEDs of the first, second, and third LEDstrings D1, D2, D3; it is further assumed that the gain gx5 of the fifthgain circuit Gx5 is equal to 3.

Arranged between the first gain circuit Gx1 and the first adder stage 53is a switch 91, controlled by the voltage generated by the firstoperational amplifier K1 of the first regulation module MREG1′,designated by V_(GM1). The switch 91 connects a corresponding input ofthe first adder stage 53 alternatively to the output of the first gaincircuit Gx1, if the first regulation module MREG1′ operates inregulation mode, or else to ground, otherwise. In this connection,considering any one of the regulation modules, it is found that, priorto entry into the regulation phase, the respective first operationalamplifier is in saturation, and thus supplies a voltage close to its ownsupply voltage; when the regulation module is in the regulation phase,the respective first operational amplifier operates outside saturation,and thus supplies a voltage intermediate between its own supply voltageand a zero voltage (on the hypothesis of unipolar supply); finally, whenthe subsequent regulation module enters the regulation phase, the firstoperational amplifier goes into negative saturation, and thus supplies azero voltage.

Arranged between the second gain circuit Gx2 and the first adder stage53 is a switch 92, controlled by the voltage generated by the firstoperational amplifier K2 of the second regulation module MREG2′,designated by V_(GM2). The switch 92 connects a corresponding input ofthe first adder stage 53 alternatively to the output of the second gaincircuit Gx2, if the second regulation module MREG2′ operates inregulation mode, or else to ground, otherwise.

Arranged between the third gain circuit Gx3 and the first adder stage 53is a switch 93, controlled by the voltage generated by the firstoperational amplifier K3 of the third regulation module MREG3′,designated by V_(GM3). The switch 93 connects a corresponding input ofthe first adder stage 53 alternatively to the output of the third gaincircuit Gx3, if the third regulation module MREG3′ operates inregulation mode, or else to ground, otherwise.

In practice, the voltage V_(Itot) is still directly proportional to theluminous flux emitted as a whole by the optoelectronic circuit 1;consequently, the mechanism of compensation of the luminous fluximplemented by the embodiment shown in FIGS. 9 and 10 is similar to theone implemented by the embodiment shown in FIGS. 2 and 6.

FIG. 11 shows, purely by way of example, the time plots of the currentsI_(D1)*, I_(D2)* and I_(D3)* that flow in the first, second, and thirdLED strings D1, D2, D3, respectively, of the embodiment shown in FIGS. 9and 10, as well as the current I_(DCOMP) that flows in the fifth LEDstring D5; in addition, FIG. 11 shows the plots of the grid voltage, ofthe total current that enters the anode of the first LED string D1, andof the luminous flux emitted as a whole by the optoelectronic circuit.

As mentioned previously, the total current follows a stepwise plot.However, thanks to the use of the switch 19 and of the switches SW1,SW2, SW3 of the regulation modules, in each step in which the voltageV_(in) is (for example) increasing, each regulation module carries outtwo regulation phases, with consequent improvement of the power factorand of the total harmonic distortion.

By what has been described and illustrated previously, the advantagesthat the present solution affords emerge clearly.

In particular, the present driving circuit enables reduction of lightflicker, without penalizing the power factor and harmonic distortion,and without the need for use of additional sensors.

Furthermore, the present driving circuit makes use of a capacitor withreduced capacitance and implements an adaptive compensation of the totalluminous flux.

In conclusion, it is clear that modifications and variations may be madeto what has been described and illustrated herein, without therebydeparting from the scope of the present invention, as defined in theannexed claims.

For example, the peak detector 22 may be of a type different from theone described. In general, the reference generator 14 may be differentfrom what has been described; for example, it may include just thevoltage divider 20, in which case the reference voltage V_(ref) does nothave a normalized amplitude.

As regards the normalization circuit 33, it may be absent, or else, ifpresent, it may be formed in a per se known manner and may possiblyperform, for example, also the function of the multiplier 26.

The transistors may be of a type different from what has been described.Further, also the circuit diagram that enables, within a regulationmodule, weighting in a different way of the detected voltage and thefeedback voltage of the subsequent module, may be different. On theother hand, instead of the detected voltage it is possible to generateany quantity indicating the current that flows in the correspondingMOSFET.

The differential amplifier of each regulation module may amplify thevoltage drop on the corresponding sensing resistor with a gain otherthan unity.

The values of the quantities mentioned in the present description maydiffer from the values given by way of example previously.

In addition, the number of regulation modules may be different from whathas been described. It is further possible for one or more of theregulation modules to include circuit components different from the onesdescribed. For example, it is possible that a low-pass filter and/or abuffer are present between the first operational amplifier and theMOSFET, in order to stabilize the electronic driving circuit. Further,the functions of the adder circuit and of the differential amplifier maybe performed by using of a circuit diagram with a single amplifier. Onceagain, between the first output terminal O₁ of the rectifier 4 andground there may be connected a capacitor (not shown) with a capacitanceof, for example, 10 nF, in order to enable a further filtering effect onthe current at input to the cascade of LED strings.

One or more of the LED strings may include two respective branches inparallel, each branch being formed by a corresponding LED string. Inthis case, the threshold voltages of the two branches may be the same asone another in order to enable correct turning-on of both of thebranches.

The number of LEDs of each LED string may be any; further, the gains ofthe gain circuits are adapted accordingly for estimating correctly theluminous flux emitted by the optoelectronic circuit.

More in general, albeit not shown, embodiments are possible in which theaforementioned first and second control loops are different from whathas been described previously, albeit continuing to envisage generationof electrical quantities indicating the total luminous flux and thevoltage present on the second terminal of the external resistor.

In addition, embodiments are possible of the type shown in FIGS. 9 and10, but in which the switch 19, the comparator 18, and the estimatorcircuit 59 are absent.

Finally, the electronic driving circuit 10 may form an electroniccircuit of an integrated type; i.e., it may be integrated in one or moredice of semiconductor material. In this case, the reference generator 14may be external with respect to the dice. Albeit not shown, and purelyby way of example, the control module 17 and the first, second, third,and fourth regulators REG1, REG2, REG3, REG4 may be integrated in thefirst die. In other words, with reference for example to the embodimentshown in FIGS. 2 and 6, the first, second, third, and fourth regulationmodules MREG1, MREG2, MREG3, MREG4 may be formed in the first die. Thecompensation module 17, the fifth regulator REG5, the comparator 18, andthe switch 19 may be formed in the second die.

The invention claimed is:
 1. An electronic circuit for driving aplurality of LED strings cascaded to one another, each LED string havinga respective cathode terminal, said electronic circuit comprising: aplurality of regulation modules; and a brightness-compensation module;wherein each regulation module is configured to be coupled to thecathode terminal of a corresponding LED string, to a resistive element,and to a capacitor coupled to the resistive element and configured to becharged through at least one portion of a current that flows in theresistive element, said regulation modules being further configured tocarry out in sequence a current-regulation phase as a function of anevolution of a rectified grid voltage; and wherein each regulationmodule is further configured so that, when said regulation modulecarries out the current-regulation phase, the regulation moduleregulates a current that flows in the corresponding LED string, in theprevious LED strings, and in the resistive element; and wherein saidbrightness-compensation module comprises: a compensation regulatorcoupled to a compensation LED string and to the capacitor; and a firstgenerator coupled to said plurality of regulation modules and to saidcompensation regulator and configured to generate a first electricalquantity indicating the luminous flux emitted by said plurality of LEDstrings and by the compensation LED string; and wherein the compensationregulator is configured to regulate a current that flows in thecompensation LED string as a function of said first electrical quantity,discharging the capacitor through the compensation LED string.
 2. Theelectronic circuit according to claim 1, further comprising a secondgenerator configured to generate a second electrical quantity indicatinga voltage drop on the capacitor; and wherein the compensation regulatoris configured to regulate the current that flows in the compensation LEDstring also as a function of said second electrical quantity.
 3. Theelectronic circuit according to claim 1, further comprising a switchstage, which is configured alternatively to: enable discharging of thecapacitor through the compensation LED string when the rectified gridvoltage is lower than a comparison voltage; and enable charging of thecapacitor by said at least one portion of the current that flows in theresistive element when the rectified grid voltage is higher than thecomparison voltage.
 4. The electronic circuit according to claim 3,wherein said comparison voltage is proportional to the sum of thevoltage drop on the capacitor and of the threshold voltage of the LEDstring that switches on first, following upon an increase in therectified grid voltage.
 5. The electronic circuit according to claim 1,wherein said brightness-compensation module comprises a processing stageconfigured to generate, as a function of the first electrical quantity,a variation signal indicating the variations of the first electricalquantity with respect to a corresponding mean value; and wherein thecompensation regulator is configured to regulate the current that flowsin the compensation LED string as a function of said variation signal.6. The electronic circuit according to claim 1, wherein each of saidregulation modules is coupled to a reference circuit configured toreceive the rectified grid voltage and to generate a reference voltagein phase with said rectified grid voltage and having an amplitudesmaller than the amplitude of said rectified grid voltage; and whereineach regulation module is configured, when carrying out thecurrent-regulation phase, to regulates the current that flows in thecorresponding LED string, in the previous LED strings, and in theresistive element so that it is directly proportional to the referencevoltage.
 7. The electronic circuit according to claim 6, wherein each ofsaid regulation modules comprises: a transistor coupled to the cathodeterminal of the corresponding LED string; a sensing circuit electricallycoupled to said transistor and configured to generate a third electricalquantity indicating a current that flows in said transistor; and acontrol stage configured to control said transistor as a function of thereference voltage and of said third electrical quantity.
 8. Theelectronic circuit according to claim 7, wherein said control stagecomprises: a first control circuit, which has a first input and a secondinput, the first input being electrically coupled to said sensingcircuit for receiving said third electrical quantity, the second inputbeing configured to receive a fourth electrical quantity indicating avoltage drop on said resistive element, said first control circuit beingconfigured to generate a feedback voltage as a function of said thirdand fourth electrical quantities; and a second control circuitelectrically coupled to the first control circuit and configured tocontrol said transistor as a function of the reference voltage and ofthe feedback voltage.
 9. The electronic circuit according to claim 8,wherein said control stage is configured in such a way that, when therespective regulation module carries out the regulation phase, saidfeedback voltage is substantially equal to the reference voltage. 10.The electronic circuit according to claim 8, wherein said second controlcircuit comprises an operational amplifier.
 11. The electronic circuitaccording to claim 8, wherein said regulation modules are electricallyconnected in sequence in such a way that the second input of the firstcontrol circuit of each regulation module other than a last one of saidregulation modules receives the feedback voltage generated by thesubsequent regulation module, the second input of the first controlmodule of the last regulation module being connected, in use, to saidresistive element.
 12. The electronic circuit according to claim 11,wherein, in each regulation module, the third electrical quantity is avoltage generated by the corresponding sensing circuit; and wherein thefirst control circuit of each regulation module other than said lastregulation module is configured in such a way that the feedback voltagegenerated by said first control circuit is a function of a weighted sumof the third electrical quantity generated by the sensing circuit ofsaid regulation module, to which a first weight is assigned, and of thefeedback voltage generated by the subsequent regulation modulesuccessive, to which a second weight is assigned, greater than the firstweight; and wherein the first control circuit of the last regulationmodule is configured in such a way that the feedback voltage generatedby said first control circuit is a function of a weighted sum of thethird electrical quantity generated by the sensing circuit of said lastregulation module, to which the first weight is assigned, and of avoltage drop on the resistive element, to which the second weight isassigned.
 13. The electronic circuit according to claim 12, wherein thefirst control circuit of each regulation module comprises an addercircuit.
 14. The electronic circuit according to claim 8, wherein thetransistor of each regulation module has a first conduction terminal anda second conduction terminal, which are respectively connected to thecathode terminal of the corresponding LED string and to a first terminalof the corresponding sensing circuit, a second terminal of saidcorresponding sensing circuit being connected to said resistive element.15. The electronic circuit according to claim 14, wherein said sensingcircuit comprises a respective differential amplifier and a respectiveresistor, which forms said first and second terminals of said sensingcircuit.
 16. The electronic circuit according to claim 6, furthercomprising said reference circuit.
 17. The electronic circuit accordingto claim 16, wherein said reference circuit comprises a voltage dividerconfigured to generate a reduced voltage as a function of said rectifiedgrid voltage.
 18. The electronic circuit according to claim 17, whereinsaid reference circuit further comprises a peak-detector circuitconfigured to generate a peak voltage, proportional to the peak value ofsaid reduced voltage; and wherein said reference circuit is configuredto generate said reference voltage so that it is proportional to theratio between the reduced voltage and the peak voltage.
 19. Theelectronic circuit according to claim 3, wherein each of said regulationmodules is configured to generate a reference voltage, which assumesalternatively a respective first voltage value, when the rectified gridvoltage is lower than the comparison voltage, or else a respectivesecond voltage value, when the rectified grid voltage is higher than thecomparison voltage, said respective second voltage value being higherthan said respective first voltage value; and wherein each regulationmodule is configured in such a way that, when said regulation modulecarries out the current-regulation phase, it regulates the current thatflows in the corresponding LED string, in the previous LED strings, andin the resistive element so that it is directly proportional to its ownreference voltage.
 20. The electronic circuit according to claim 19,wherein said brightness-compensation module comprises a processing stageconfigured to generate, as a function of the first electrical quantity,a variation signal indicating the variations of the first electricalquantity with respect to a corresponding mean value; and wherein thecompensation regulator is configured to regulate the current that flowsin the compensation LED string as a function of said variation signal.